A number of prior art processes have been proposed for the preparation of oxalate esters by the oxidative carbonylation of alcohols in the presence of metal salt catalysts, dehydrating agents and ferric or cupric redox agents in solution.
The present invention is directed to a process for the preparation of oxalate esters in high yield and avoiding the problems associated with the prior art processes of carbonylating alcohols directly to obtain the desired oxalate ester. More particularly, the present process relates to the synthesis of oxalates by reacting carbon monoxide, a particular amount of an alcohol, and oxygen with an enol ether under elevated temperature and pressure conditions in the presence of a catalytic amount of a palladium, platinum, cadmium, cobalt, rhodium, zinc or copper salt catalyst and at least a catalytic amount of an amine base and includes the employment of catalytic amounts of copper (II) or iron (III) oxidant salts in addition to catalytic amounts of an ammonium or substituted ammonium salt compound and ligands of the metal salt catalysts.
U.S. Pat. No. b 3,393,136 describes a process for the preparation of oxalates by contacting carbon monoxide at superatmosheric pressure, with a saturated monohydric alcohol solution of a platinum group metal salt and a soluble ferric or cupric salt (redox agent) while maintaining the salts in a highly oxidized state by the simultaneous introduction of oxygen or the application of a direct current electrical potential to the reaction zone. When oxygen is employed, explosive mixtures of oxygen and combustible organic vapors in the gas phase must be avoided and water scavengers or dehydrating agents such as alkyl orthoformic acid esters must be added to the liquid phase to prevent the accumulation of water.
In a recent article by Donald M. Fenton and Paul J. Steinwand, Journal or Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704, a general mechanism for the oxidative carbonylation of alcohols to yield dialkyl oxalates using a palladium redox system, oxygen and dehydrating agents has been proposed. In the absence of the necessary dehydrating agent, a large amount of carbon dioxide is formed and oxalates are not produced. The necessity of the iron or copper redox system during the oxalate synthesis is emphasized.
A recent West German Pat. No. 2,213,435 discloses a method for the synthesis of oxalic acid and oxalate esters in water and alcohol respectively. A platinum group metal salt, a salt of a metal more electropositive than the platinum group metal, e.g. coppper (II) chloride and an alkali metal salt comprise the catalyst. Oxygen in stoichiometric amounts was employed as the oxidant. A disadvantage of such reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect reaction. Uner non-explosive conditions only trace amounts of oxalate can be obtained.
A more recent West German Patent No. 2,514,685 describes a process for the production of dialkyl oxalates by reacting an aliphatic alcohol with CO and oxygen under pressure in the presence of a catalyst of a mixture of a salt of a metal from the platinum group and a salt of copper or iron and an accelerator including nitrates, sulfates, carbonates, tertiary amines and hydroxides and carboxylates of alkali metals and alkaline earth metals. Conversion of the alcohol employed to the dialkyl oxalates in such process is low.
Many important commercial applications have been developed for the oxalate products of this invention, for example, as cellulose ether or ester and resin solvents, as dye intermediates and the preparation of pharmaceuticals.
The process of the present invention provides a method of carrying out the oxidative carbonylation of enol ethers to produce an oxalate ester without the coproduction of water which acts to poison the catalyst system and which even in small amounts also causes the production of large quantities of carbon dioxide and an attendant loss of the desired oxalate ester. Thus, by the process of the present invention, only very small concentrations of water can accumulate in the reaction system since by the mechanism of the reaction any water which might be formed is rapidly consumed upon formation of a coproduct ketone or aldehyde. In addition, the coproduction of carbonate esters associated with such carbonylation reactions is minimized giving excellent selectivities to oxalate esters with high conversions of the enol ether. The ketone or aldehyde coproduced with the desired oxalate ester by the oxidative carbonylation reaction of the enol ether may be readily separated from the desired oxalate and converted back to the respective reactant enol ether.
Other advantages of the present invention, as compared to known prior art processes for the production of oxalates are (1) elimination of hazardous operational conditions by avoiding exposive mixtures of oxygen and carbon monoxide, (2) avoiding the use of large amounts of corrosive chloride ions (3) ease of recovery and regeneration of the metal salt catalysts for reuse in the process and (4) the ability to employ in the process as catalysts the more readily available copper salts and other metal salts in place of the more expensive platinum group metal salts.